The present invention relates to condensation on surfaces. More specifically, the invention relates to implantation of ions on a surface to adjust its wetting characteristics, e.g., adjustment of dropwise condensation.
If pure vapor touches a solid wall surface whose temperature is below the saturation temperature of the vapor, the vapor condenses on this surface as a liquid. The condensation process can be carried out in two different forms, as film condensation or as dropwise condensation. In the case of film condensation, the wall surface is completely wetted by the condensate, forming a connected condensate film. In contrast to film condensation, the condensate does not wet the wall completely in dropwise condensation; the condensate forms independent drops. The wettability of the surface is responsible for the formation of each type of condensation and has a very strong effect on the performance of a possible heat transfer process. Thus, a heat transfer coefficient of four to ten times larger has been measured for dropwise condensation of water vapor [1-3], which constitutes its great technological importance.
Although conditions for promoting dropwise condensation have been known in principle for several decades and experiments with coatings as promoters have been carried out successfully at least in part, the application of dropwise condensation is still in a testing phase. The main problems for the realization of dropwise condensation are that the working boundary surface phenomena such as complete or incomplete wettability are insufficiently theoretically described and strongly depend on influences caused in the practical operation by contamination, oxidation of the surface, adsorption layers, and gas enclosures. Additionally, surface defects (roughness) and chemical pollutants cause the frequently observed hysteresis of the wetting angle, which has an unfavorable effect on heat transfer.
Ion plantation or ion implantation of N, Ar, He, H, Cr, Fe, and Al in copper tubes was successfully used to achieve dropwise condensation by Zhang, Zhao et al. [4-6], but the experiments with Sb, Sn, In, Se, and Bi failed. Later on, F+ and C+ (produced from Teflon irradiation), along with Cr+ or Cr+ alone, were implanted on PTFE coated surfaces [7]. The method of the simultaneous plantation or implantation of Cr+ with other ions was tested with N+ and also with CH4 [8] and C2H6 [9], with different alloys being produced in each case. For all these methods, it is typical and thus is attributed to the particular methods that
these alloys can only be produced by several somewhat complex manufacturing steps connected in series, as described in great detail by Zhao and Burnside [7]. These processes consist of work steps for the removal of pollutants and oxides, surface cleaning by sputtering with Ar+, N+, P+ and/or Cr+ and N+ simultaneously, and subsequent plantation and/or implantation steps;
high energy doses of approximately 100 keV [7] with dose concentrations of  greater than 1017 cmxe2x88x922 are needed for the sputtering and ion implantation processes (with the exception of the first attempts with ion plantation [4] which, however, represents only a surface coating, and not an ion implantation, with an alloy formation due to heat influences;
alloys of different types are produced, partly consecutively, with a total layer structure thickness of up to 5 xcexcm;
the dopant elements and the dosages that can be used were selected in a purely empirical manner without any theoretical basis, and which solely state with regard to dosing that a high dose improves the tendency of drop formation [10].
With this invention, a method is introduced and presented that is a substantial simplification of the methods described thus far and which allows, e.g., by means of the use of technologically established metallic base materials, a low-cost production of heat transfer systems having suitable surfaces that are usable on a large-scale in which selection of dopant elements and the necessary dose concentration can be determined theoretically.
The process of dropwise condensation is adjustable in a temporally and locally stable manner by providing a chemical potential which works with the same magnitude against the chemical potential being generated during drop formation. This new method allows the identification of the necessary dose concentration and the suitable type of dopant elements for ion implantation of surfaces used for selected adjustment of dropwise condensation.
Thus, according to the present invention:
formation of oxides before actual ion implantation process is favorably permitted and thus ion implantation is executed directly without any cleaning processes beforehand and without pre-sputtering;
in a large process window with a broadband ion source, basically the simplest manageable element, nitrogen, is used for ion implantation, with the use of mixed ions (N2+, N+, N++) providing the necessary ion concentration and accelerating the process (by a factor of 10);
an ion concentration of 1015 cmxe2x88x922 (based on theoretical considerations) is regarded as basically sufficient, which allows a small irradiation energy and thus produces no additional surface layers, leading to more favorable boundary conditions for the heat transfer. A minimum dose concentration of 1015 cmxe2x88x922 results from calculation of the distances of dopant elements for the adjustment of dropwise condensation. By increasing this dose concentration, the intensity of dropwise condensation, and thus the level of the heat transfer performance, is directly increased;
further simplifying the process, often only a limited part of the surface is ion implanted, forming afterwards a surface area that is completely useable for dropwise condensation by balancing out the dose concentration gradient by horizontal diffusion inside the base material along the surface,
directed to different areas of application, favorably different base materials can be used for ion implantation, resulting in fairly substantial advantages in costs or process execution, e.g., hard-chromium-plated copper, chemically nickel-plated copper; or rust-proof metals; such as high-grade steel; aluminum; or titanium;
the intensity of dropwise condensation can be adjusted by selection of dopant elements on the basis of theoretical considerations using the ratio of the dipole moments of the dopant elements with that of nitrogen.